Ophthalmic, and other types of lenses are typically produced from lens blanks of glass or plastic having two major surfaces, one of which is typically finished, and the other of which is unfinished. Cutting, polishing, and fining operations are performed on the unfinished surface of the lens blank by a machine responsive to data corresponding to a particular lens prescription. The cutting operations are usually accomplished by employing a ball mill for plastic lenses, or a grinder for glass lenses. These cutting operations generally create a lens surface closely approximating the shape of the finished lens. However, the cut surface of the lens blank is often rough and requires that subsequent polishing and fining operations be performed on the lens blank to achieve the requisite optical clarity.
The polishing and fining operations are ordinarily performed by engaging the cut surface of the lens blank with an abrasive surface having a shape that closely approximates the desired finished shape of the lens as defined by the lens prescription. This abrasive surface is referred to by those skilled in the pertinent art as a tool or "lap." During operation, the device to which the lens blank is mounted, moves the blank over the abrasive surface of the lap along a conforming contoured semi-spherical path, thereby polishing and/or fining the lens surface. Laps generally consist of two main components, a mounting surface or mandrel, and a removable abrasive pad that mounts on the mandrel and against which the lens blank is moved during polishing and fining operations. The shape of the mandrel must conform as closely as possible to the prescribed shape of the lens, therefore, different lens prescriptions require different laps to be used.
During polishing and fining operations, it is often necessary to lift the lens blank off of the lap and rinse the abrasive pad to remove lens material in the form of particulate that has built-up during the polishing and/or fining operations. To prevent the abrasive pad from separating from the mandrel during rinsing a releasable adhesive is used to bond the pad to the mandrel. A difficulty associated with adhesively attaching the abrasive pad to the mandrel is that after extended periods of use it is often necessary to change abrasive. The adhesive can make separating the abrasive pad from the mandrel difficult and time consuming. In high production situations where abrasive pads are regularly replaced, significant amounts of time can be lost separating the abrasive pad from the mandrel, thereby adding to the time and expense associated with preparing lenses.
Another difficulty associated with known lens surface generating machinery is that a significant amount of time consuming manual fixturing and set-up prior to beginning a polishing and/or fining operation is usually required. Moreover, the assembly to which the lens blank is mounted is generally comprised of a combination of different parts stacked in series, one-on-top-of-the-other with each part having a machining tolerance associated with it which usually results in the part deviating slightly from its ideal size and geometric configuration. This creates small dimensional differences between mating parts that are unpredictable and which must be compensated for when preparing the machine to generate a lens surface on a lens blank. In addition, the additive nature of these dimensional differences can result in a loss in the overall mechanical stiffness of the machine which translates into reduced machining accuracy. Furthermore, the number of stacked parts and the serial nature of their assembly, usually results in relatively large moving masses that have significant inertia, making precise movement of the lens blank relative to the cutting tool difficult.
Still another difficulty associated with known prior art lens surface generating machinery is that they are typically three axis machines, and as such are limited with respect to the patterns of motion that can be achieved when moving the lens over the abrasive surface of the lap. This limitation on movement often results in a less than ideal finished lens surface. Moreover, the pressure exerted on the lens blank by these known machines is generally not uniform across the surface of the lens, often causing distortions at the periphery of a finished lens.
Based on the foregoing, it is the general object of the present invention to provide a lens surface generating apparatus that overcomes the above-described drawbacks of prior art lens surface generating machines.
It is a more specific object of the present invention to provide a lens surface generating apparatus that does not require extensive manual set-up operations.
It is a further object of the present invention to provide a lens surface generating apparatus whereby an eyeglass manufacturer does not have to maintain a large inventory of laps.